This application is based upon and claims benefit of priority of Japanese Patent Application No. 2000-238833 filed on Aug. 7, 2000, the content of which is incorporated herein by reference.
1. Field of the Invention
The present invention relates to a gas concentration detector that detects constituent gas concentration in exhaust gas from an internal combustion engine. The gas concentration detector detects, for example, oxygen concentration in the exhaust gas to control an air-fuel ratio in an intake system of the internal combustion engine. More particularly, the present invention relates to a controller that controls temperature of a detector element of the gas concentration detector.
2. Description of Related Art
A gas concentration detector of this kind is the known marginal-current-type oxygen sensor. A device for controlling a heater of such a sensor is disclosed in, for example, JP-A-278279 and JP-A-300716. Two types of such sensors have been known, one is a cup-type and the other is a lamination-type having laminated heater and sensor elements. Recently, the lamination-type oxygen sensors are becoming more popular in the market, because they can be made compact at low cost and have better temperature characteristics.
An example of the lamination-type oxygen sensor is disclosed in JP-A-11-344466. A sensor element and a heater for heating the sensor element to activate the sensor element are positioned closely to each other, and therefore a temperature difference between the sensor element and the heater is relatively small. Therefore, a current supplied to the heater is controlled based on an internal impedance of the sensor element, not based on a detected heater resistance. That is, the heater current is controlled by feeding-back the sensor element impedance so that the sensor element impedance is maintained at a target value. In the lamination-type oxygen sensor, an amount of heat generated in the heater, i.e., an amount of heater current, can be kept low, because the heat is effectively transferred from the heater to the sensor element.
In the conventional oxygen sensor, however, it is highly possible that excessive heater current is supplied to the heater when the heater resistance is low at a low temperature. As shown in FIG. 15A, the heater resistance increases in proportion to the heater temperature, and therefore the lower the heater temperature, the lower the heater resistance. Accordingly, as shown in FIG. 15B, the power supplied to the heater exceeds a permissible level at a low heater temperature. Since the power supply to the heater commences, in the conventional sensor, after the internal combustion engine is cranked and brought into operation, an excessive current is supplied to the heater if the heater temperature is low. Moreover, a high current is supplied to the heater at the beginning of the engine operation to quickly activate the sensor element. If the power exceeding the permissible level is supplied to the heater, the heater may be broken by an excessive heat stress and also the sensor element may be broken thereby. This problem is serious especially in the lamination-type oxygen sensor.
Further, the heater resistance is not detected in the lamination-type sensor because the temperature difference between the heater and the sensor element is not large. Therefore, the heater current cannot be controlled according to the heater resistance, and it is highly possible that the heater power exceeding the permissible level is supplied to the heater at a low temperature. It is conceivable to estimate the heater temperature based on an engine coolant temperature, an intake air temperature or the like and to prohibit the power supply to the heater at a very low temperature. However, there is a possibility that the heater temperature is low even if the coolant or the intake air temperature is relatively high. Such a situation occurs, for example, when the engine is re-started after a dead soak. Accordingly, it is difficult to solve the excessive power supply problem only by measuring the coolant or the intake air temperature.
The present invention has been made in view of the above-mentioned problem, and an object of the present invention is to provide an improved gas concentration detector in which a detector element is properly activated by a heater while avoiding an excessive heating power supply to the heater.
The gas concentration detector detects concentration of a constituent gas such as oxygen in exhaust gas emitted from an internal combustion engine. An air-fuel ratio in an intake system is controlled based on the detected gas concentration. The gas concentration detector includes a detector element made of a material such as solid electrolyte and a heater to activate the detector element.
During a cranking period of the engine, the heater is pre-heated under a pre-heating control, and after the engine cranking is completed, the heater is controlled under an in-operation control. At the beginning of the in-operation control, heating current is supplied to the heater with a full duty ratio. Thereafter, the current is supplied with a controlled duty ratio to keep the detector element activated. Since a heater resistance is increased to a certain level by the pre-heating, the heater current is limited to a certain level when the heater current is supplied with the full duty ratio at the beginning of the in-operation control.
In the pre-heating control, the duty ratio of the heater current supply is controlled according to engine starting conditions such as an engine coolant temperature or an intake air temperature. The duty ratio is controlled to supply a higher amount of current as the coolant temperature becomes lower. Further, the duty ratio is adjusted according to a battery voltage to supply a substantially constant power to the heater. Alternatively, an impedance of the detector element is detected, and the duty ratio is controlled to bring the impedance to a target level. The detector element is pre-heated to a temperature (e.g., 500xc2x0 C.) which is lower than a temperature at which the detector element is activated (e.g. 750xc2x0 C.) Preferably, the duty ratio in the pre-heating control is gradually increased to the full duty ratio at the beginning of the in-operation control. The pre-heating may be performed only when the engine is started at a low temperature.
According to the present invention, an excessive current supply to the heater, which otherwise occurs at the beginning of the in-operation control, is surely avoided even when the engine is started at a very low temperature because the heater resistance is increased to a certain level by pre-heating in the cranking period.
Other objects and features of the present invention will become more readily apparent from a better understanding of the preferred embodiments described below with reference to the following drawings.